score = input('请输入你的成绩')
num_score = int(score)
if num_score >= 90:
    print('a')
elif num_score >= 80:
    print('b')
elif num_score >= 70:
    print('c')
elif num_score >= 60:
    print('d')
elif num_score =< 60:
    print('e')
